It has generally been believed that loss of neurons in the adult human brain—as it occurs in aging humans and in neurological disorders—is an irreversible process. Many major diseases of the human brain involve deficiencies of select neuronal populations. The inability by the adult human brain to generate replacement cells is thought to be a leading cause for the irreversible and progressive nature of several neurological diseases and is responsible for persistent and ongoing impairment. In most regions of the human brain, the generation of neurons is generally confined to a discrete developmental period. After this developmental period, it believed that no further generation of brain cells occurs in the living human brain.
Exceptions to this general rule exist in specific regions of the adult mammalian brain. The dentate gyrus of the hippocampus and the subventricular zone have been shown to generate new neurons well into the postnatal and adult period. For example, in the rodent brain, granule neurons may be generated throughout life from a population of continuously dividing progenitor cells residing in the subgranular zone of the dentate gyrus. It is likely that the human brain may also enjoy these regenerative features.
Attempts have been made to learn more about possible neurogenesis in the adult human brain. For example, scientists have dissected human brain tissue from postmortem patients to achieve neurogenesis. Unfortunately, however, the genesis of new neurons in situ in the living adult human brain and methods to enhance, control or modulate this process have not yet been demonstrated. Accordingly, attempts have been made to prevent or slow down neurodegeneration of the human brain. For example, U.S. Pat. No. 5,683,422 discloses techniques for treating neurodegenerative disorders by electrical brain stimulation. Similarly, U.S. Pat. No. 5,707,396 describes methods of arresting degeneration of the neurons by high frequency stimulation.
It is therefore desirable to provide a technique for inducing neurogenesis (namely, the producing of new or replacement neurons) within a living brain of an adult human.